The present invention relates to methods and apparatuses for characterizing or evaluating the strength or effectiveness of oil-based drilling fluids for use in drilling well bores in subterranean formations. The invention relates particularly to methods and apparatuses for characterizing water-in-oil or invert emulsion fluids for use in drilling well bores in hydro-carbon bearing subterranean formations.
Drilling fluids are frequently used in oil and gas drilling operations. These fluids serve many purposes including, but not limited to, removing the cuttings produced, lubricating and cooling the drill bit, and supporting the walls of the hole being drilled. Oil-based drilling fluids are frequently used when drilling wells for oil and gas. These oil-based drilling fluids are typically water-in-oil emulsions that are stabilized with the addition of emulsifiers. The water phase is usually about 5% to about 40% of the total liquid volume and is usually comprised of (but not limited to) calcium chloride brine. If the formulation of the invert emulsion drilling fluid becomes unbalanced, due to, for example (without limitation), contamination, improper product additions, or thermal degradation, the oil-based drilling fluids tend to revert to an oil-in-water emulsion. As a result of the reversion to an oil-in-water emulsion, the water becomes the continuous phase and can cause the solids in the drilling fluid to become water wet. Such wetting has significant negative consequences to a drilling operation and requires expensive remedial action in order to prevent the loss of the well.
Electrical Stability measurements give an indication of the stability of a water-in-oil emulsion. In order to characterize the stability of a water-in-oil emulsion, a strong electric field is applied across a small gap between two electrodes. Normally, alternating current (AC) electrical fields are applied to mitigate damage to the surfaces of the electrodes. The gap is filled with a sample of the fluid to be tested. The electric field is increased until significant current flows through the sample between the electrodes. Oil based drilling fluids with emulsified brine are essentially electrical insulators to weak electric fields. As the field strength increases, the emulsified droplets of water in the electrode gap begin to elongate and align with the electric field. If the electric field is sufficiently strong, the droplets may eventually merge, forming a conductive bridge across the electrode gap. The droplet bridge can conduct a significant electric current. The moment that the current exceeds a specified trip current is described as breakdown.
It has been demonstrated that the field strength at breakdown, measured in peak volts across the electrode gap, is related to the stability of the emulsion. (Growcock F B, Ellis C F and Schmidt D D: “Electrical Stability, Emulsion Stability, and Wettability of Invert Oil-Based Muds,” SPE Drilling & Completion 9, no. 1 (March 1994): 39-46.) The peak voltage required to cause the breakdown is defined as Electrical Stability of the oil-based drilling fluid. The higher the peak voltage at which this breakdown occurs, the greater is the Electrical Stability of the fluid being tested. The American Petroleum Institute's “Recommended Practice Standard Procedure for Field Testing Oil-Based Drilling Fluids”, API Recommended Practice 13B-2, Third Edition, February 1998 (“the API Procedure”), is incorporated herein by reference. Paragraph 8.1.1 of the API Procedure defines the Electrical Stability of an oil-based drilling fluid as “the voltage in peak volts-measured when the current reaches 61 μA.”
The Electrical Stability measurement is affected by the electrode configuration, AC frequency, gap width, trip current, and the rate of increase of the electrical field. All of these parameters have been specified in the procedures of the American Petroleum Institute (API) for use in the industry. The API Procedure requires that before an Electrical Stability measurement, the sample fluid should be stirred for 10 seconds.
However, the Electrical Stability, as measured by current manual methods, is known to be sensitive to operator methodology and many components of the drilling fluid. Additionally, typical methods of measuring Electrical Stability of oil-based drilling fluids do not account for the effects of fluid gelation over time. Such pitfalls make current methods of measuring Electrical Stability prone to errors and, therefore, less useful.
Such problems are further compounded by how time-consuming current methods are. The typical methods yield very few data points to distinguish trends. Additionally, traditional Electrical Stability measurements are relatively noisy due to the non-homogeneous nature of drilling fluids.